1. Field of the Invention
The present invention relates generally to a changeable chuck system which is changeably mounted on a main spindle of a machine tool. More specifically, the invention relates to a changeable chuck system including a chuck, which can grip and release a workpiece by itself and which can be detached from a chuck mounting portion, such as the tip of a main spindle of a machine tool or a fixed workpiece-handling position other than the main spindle, to be transferred to another chuck mounting portion while gripping the workpiece.
2. Description of the Prior Art
A chuck system of this type is disclosed in Japanese Patent Appln. Laid-Open No. 5-253713 published on Oct. 5, 1993. In this system, a chuck is mounted on a chucking jig provided at a location other than the tip of a main spindle of a machine tool. When a fluid communication between a fluid connection port on the side of the chucking jig and a fluid supply/discharge port on the side of the chuck is established, pressurized fluid is supplied from the side of the chucking jig to a cylinder chamber of a chuck-jaw operating cylinder in the chuck via a check valve, so that the cylinder is actuated to open and close chuck jaws. The check valve prevents the fluid supplied to the cylinder chamber from being discharged to maintain the pressure of the supplied fluid so as to maintain the workpiece gripping state in which the workpiece is gripped by the chuck. While the workpiece gripping state is maintained, the chuck is detached from the chucking jig, and then, the chuck is mounted on the main spindle of the machine tool. Then, at the tip of the main spindle serving as a chuck mounting portion, a pull rod, which passes axially through the main spindle and which is linked to a rotating cylinder connected to the back end of the main spindle, is engaged with a piston rod which projects backwards from a piston of the chuck-jaw operating cylinder provided in the chuck. While the workpiece is mounted on the main spindle, if the workpiece gripping force is not sufficient with only the fluid pressure maintained by the check valve, the rotating cylinder forces the pull rod to draw the piston of the chuck-jaw operating cylinder to increase the gripping force for safety.
Another chuck system is disclosed in Japanese Utility Model Publication No. 62-28324 published on Jul. 21, 1987. In this system, pressurized fluid is supplied to the interior of a chuck from a supply port provided in the outer periphery of the chuck. By a check valve provided in the chuck and an air chamber (an accumulator chamber). the pressurized fluid is confined in a cylinder chamber provided in the chuck. In this state, the chuck is prepared on a workpiece mounting setup portion other than a main spindle of a machine tool, and an opening rod provided on the workpiece mounting setup portion pushes a release shaft projecting at the rear side of a piston of a cylinder in the chuck to move the piston to open chuck jaws. After a workpiece is introduced between the chuck jaws, the opening rod is moved backwards, and the workpiece is gripped by the chuck jaws, which are closed by the force of a spring for biasing the piston in a gripping direction and by the confined high-pressure fluid. The chuck thus gripping the workpiece is detachably mounted on the main spindle of the machine tool by means of a pull rod provided on the main spindle. After the machining is completed by means of the machine tool, the pull rod pushes the release shaft and the workpiece is removed from the main spindle. Thereafter, the pull rod is released and the chuck is detached from the main spindle.
Another chuck system is also disclosed in Japanese Utility Model Appln. Laid-Open No. 59-5210 published on Jan. 13, 1984. In this system, two arms are provided on a pivot which is pivotable and movable forwards and backwards in an axial direction. A chuck gripping mechanism is provided on each of the tips of the arms. A self-hold type chuck, such as a front chuck, is gripped by the chuck gripping mechanism to be moved to a mounting position of a main spindle, and then, the chuck is mounted on the main spindle by means of a connecting mechanism. In this mounted state, the machining is performed.
Still another chuck system is disclosed in Japanese Patent Appln. Laid-Open No. 62-193710 published on Aug. 25, 1987. In this system, chuck mounting means is provided on the front portion of a main spindle. A chuck is mounted on the main spindle by the chuck mounting means. In this mounted state, a jaw operating shaft is moved by means of a draw bar to open and close jaws.
In the case of the aforementioned chuck system disclosed in Japanese Patent Appln. Laid-Open No. 5-253713, irrespective of the fluid supplied to the jaw operating cylinder, when the chuck is mounted on the chuck mounting portion other than the main spindle, the jaw operating cylinder in the chuck is operated by the fluid to open and close the jaws. On the other hand, when the chuck is mounted on the main spindle, no pressurized fluid is supplied to the jaw operating cylinder in the chuck, and the gripping force of the jaws is increased (the jaws are further moved in the gripping direction) using the driving force of a separate rotating cylinder. Therefore, when the chuck is mounted on the main spindle, cylinders of two systems must be used to move the jaws in the gripping direction, so that there are cylinders that are not fully used. In addition, when the pressurized fluid operating the jaw operating cylinder in this chuck system is a pressure oil, if the pull rod draws the piston rod of the jaw operating cylinder on the main spindle to increase the gripping force, the pressure oil confined in the cylinder chamber of the jaw operating cylinder comes to have a negative pressure, so that the check valve preventing reverse flow of the pressure oil is opened to allow the fluid to leak. Therefore, when the drawing of the pull rod is released after the machining is completed on the main spindle mounting thereon the chuck, the pre-established pressure in the jaw operating cylinder becomes less than that before the piston rod is drawn by the pull rod, so that there occurs a problem in that the workpiece gripping force can not constantly be maintained. Moreover, in this system, when the pressure oil is charged while the chuck is mounted on the main spindle, even if it is tried to move the piston in the jaw releasing direction, the piston can not move since the pressure oil is incompressible. Therefore, there is a disadvantage in that the workpiece can not substantially be released when the chuck is mounted on the main spindle.
In the case of the aforementioned chuck system disclosed in Japanese Utility Model Publication No. 62-28324, an air chamber in the case of a pneumatic system or an accumulator chamber in the case of a hydraulic system is installed in the chuck system. Therefore, even when the chuck is mounted on the main spindle, the fluid can be caused to flow into the air or accumulator chamber by pushing the release shaft, so that the workpiece can be removed. However, when it is intended to increase the gripping force while the chuck is mounted on the main spindle, or when it is intended to supply the fluid to the interior of the chuck during the rotation of the main spindle, high-pressure fluid must be supplied from the supply port again, so that an additional fluid piping must be connected to the supply port while the chuck is mounted on the main spindle.
In the case of the aforementioned chuck system disclosed in Japanese Utility Model Appln. Laid-Open No. 59-5210, the self-hold type chuck is mounted on the main spindle, and the jaws can not be opened and closed while the chuck is mounted on the main spindle. Therefore, there are problems in that the workpiece can not be removed and the gripping force can not be increased at the position of the main spindle.
In the case of the aforementioned chuck system disclosed in Japanese Patent Appln. Laid-Open No. 62-193710, although the jaws can be opened and closed while the chuck is mounted on the main spindle, there is a problem in that when the chuck is detached from the main spindle, the jaws are opened, so that the workpiece can not be gripped.